CoW - don't try to update underlying values of Series/column inplace for inplace operator (#55745)

Author: jorisvandenbossche

pandas/core/generic.py

@@ -12378,7 +12378,12 @@ def _inplace_method(self, other, op) -> Self:
 
         result = op(self, other)
 
-        if self.ndim == 1 and result._indexed_same(self) and result.dtype == self.dtype:
+        if (
+            self.ndim == 1
+            and result._indexed_same(self)
+            and result.dtype == self.dtype
+            and not using_copy_on_write()
+        ):
             # GH#36498 this inplace op can _actually_ be inplace.
             # Item "ArrayManager" of "Union[ArrayManager, SingleArrayManager,
             # BlockManager, SingleBlockManager]" has no attribute "setitem_inplace"

pandas/tests/copy_view/test_methods.py

@@ -1815,12 +1815,22 @@ def test_update_chained_assignment(using_copy_on_write):
         tm.assert_frame_equal(df, df_orig)
 
 
-def test_inplace_arithmetic_series():
+def test_inplace_arithmetic_series(using_copy_on_write):
     ser = Series([1, 2, 3])
+    ser_orig = ser.copy()
     data = get_array(ser)
     ser *= 2
-    assert np.shares_memory(get_array(ser), data)
-    tm.assert_numpy_array_equal(data, get_array(ser))
+    if using_copy_on_write:
+        # https://github.com/pandas-dev/pandas/pull/55745
+        # changed to NOT update inplace because there is no benefit (actual
+        # operation already done non-inplace). This was only for the optics
+        # of updating the backing array inplace, but we no longer want to make
+        # that guarantee
+        assert not np.shares_memory(get_array(ser), data)
+        tm.assert_numpy_array_equal(data, get_array(ser_orig))
+    else:
+        assert np.shares_memory(get_array(ser), data)
+        tm.assert_numpy_array_equal(data, get_array(ser))
 
 
 def test_inplace_arithmetic_series_with_reference(